1. Field of the Invention
This invention relates to a communication method, a transmitter, a receiver, and a cellular radio communication system, and more particularly, is applicable to a portable telephone system.
2. Description of the Related Art
Conventionally, in the field of radio communication, a combination of high priority data and low priority data is generally transmitted. Such a typical digital radio communication system that a combination of high priority data and low priority data is transmitted will be explained below. In the following explanation, one frame of transmission data is composed of the high priority data and the low priority data, and the transmission data for one frame is transmitted by one transmission slot. Note that frame means data units in the processing of digital data, and slot means data units in the transmitting of digital data.
In this radio communication system, two types of information are transmitted by one frame as shown in FIG. 1, so as to divide one frame into a high priority field and a low priority field. The high priority data is stored in the high priority field and the low priority data is stored in the low priority field. In this case, not only information bits of the data are to be stored, but also error detection and correction bits are stored in each field. Thereby, the receiving side can detect and correct errors of the received information bits by using the error detection and correction bits.
In addition, the error detection and correction bits having superior ability to detect and correct errors are generally added to the high priority data, and the error detection and correction bits having a comparatively inferior ability to detect and correct errors are added to the low priority data. For this reason, the error detection and correction bits added to the high priority data tend to be longer in its size.
Here, FIG. 2 shows a transmitter for actually transmitting a frame having such structure. As shown in FIG. 2, in the transmitter 1, a bit stream DH1 composed of the high priority data is firstly input to a first error correction bit adding circuit 2, and a bit stream DL1 composed of the low priority data is input to a second error correction bit adding circuit 3.
The first error correction bit adding circuit 2 calculates the error detection and correction bits based on the input bit stream DH1 and adds these error detection and correction bits to the bit stream DH1 so as to generate a bit stream D1 which is stored in the aforesaid high priority field. The bit stream D1 is output to a frame generation circuit 4 at the subsequent stage. In connection, in the first error correction bit adding circuit 2, the error detection and correction bits having superior ability to detect and correct errors are calculated and added.
While, the second error correction bit adding circuit 3 calculates the error detection and correction bits based on the input bit stream DL1 and adds these error detection and correction bits to the bit stream DL1 so as to generate a bit stream D2 which is stored in the aforesaid low priority field. The bit stream D2 is output to a frame generation circuit 4 at the subsequent stage. In connection, in the second error correction bit adding circuit 3, the error detection and correction bits, which have inferior ability to detect and correct errors to the first error correction bit adding circuit 2, are calculated and added.
As shown in FIG. 2, the frame generation circuit 4 adds the bit stream D2 to the end of the bit stream D1 to generate transmission data stream D3 for one frame, which is output to a modulation circuit 5. The modulation circuit 5 modulates the transmission data stream D3 to generate transmission symbol stream D4, which is output to a transmission circuit 6. After performing the filtering processing on the transmission symbol stream D4, the transmission circuit 6 performs the digital-to-analog conversion processing to generate a transmission signal, and then performs the frequency conversion processing on the transmission signal to generate a transmission signal S1. This transmission signal S1 is transmitted via an antenna 7, so that the transmitter 1 transmits a combined data of the high priority data and the low priority data.
On the other hand, as shown in FIG. 3, in a receiver 10, the transmission signal S1 transmitted from the transmitter 1 is received at an antenna 11, and is input to a reception circuit 12 as a reception signal S2. The reception circuit 12, after performing the filtering processing on the reception signal S2, performs the frequency conversion processing on the reception signal S2 to take out a baseband signal, and performs the analog-to-digital conversion processing on the baseband signal to take out a reception symbol stream D5.
The demodulation circuit 13 performs a predetermined demodulating processing on the reception symbol stream D5 taken out by the reception circuit 12 to decode a reception data stream D6 (the reception data stream D6 is not completely equal to the transmission data stream D3, and includes data error received through transmission), which is output to a field division circuit 14. The field division circuit 14 divides the decoded reception data stream D6 into a bit stream D7 of the high priority field and a bit stream D8 of the low priority field, and outputs these bit streams D7 and D8 respectively to a first error detection and correction circuit 15 and a second error detection and correction circuit 16.
The first error detection and correction circuit 15 detects data errors included in the received information bits based on the error detection and correction bits included in the bit stream D7 and corrects the data errors, so that the transmitted information bits are decoded, and output as a bit stream DH2 of the high priority data. In the same way, the second error detection and correction circuit 16 detects data errors included in the received information bits based on the error detection and correction bits included in the bit stream D8 and corrects the data errors, so that the transmitted information bits are decoded, and output as a bit stream DL2 of the low priority data. By this processing, in the receiver 10, the high priority data and the low priority data are respectively decoded from the reception signal S2.
Transmitting a combination of the high priority data and the low priority data is also performed by the cellular radio communication system such as a portable telephone system. This point will be concretely explained below with an example of a portable telephone system.
Generally, in a portable telephone system, the area where communication service is provided is divided into cells of desired size, and a base station as a fixed radio station is positioned in each cell. A portable telephone device as a mobile radio station radio-communicates with the base station in the cell where the device exists, and so-called cellular radio communication system is constructed.
In such a portable telephone system, when a calling is made by a portable telephone device for example, the call processing is performed by the procedure explained below. The portable telephone device firstly transmits control data composed of preamble data and message data to a base station using a control channel called a random access channel (RACH). The base station monitors the random access channel and detects the existence of the preamble data to detect whether there is a message from the portable telephone device or not. When the preamble data is detected, the base station judges that there is a message from the portable telephone device, detects the following message data, and analyzes the contents of the message data. If the content of the message data is a call request, the base station determines a demesne control channel (DCCH) to be used for communication with the portable telephone device, and informs of this channel number to the portable telephone device via an answer governing channel (AGCH). A predetermined control processing is executed between the portable telephone device and the base station via the informed occupational control channel, so that a call processing from the portable telephone device is realized.
In the call processing, the control data initially sent from the portable telephone device is composed of preamble data and message data, as described above. In this case, the preamble data represents the existence of a message data. The base station side firstly detects the existence of the preamble data to detect whether a message data exists or not. Accordingly, in case that the base station side prioritizes the data based on the detection order, the preamble data has the highest priority and the message data showing the contents of a concrete request has lower priority than that of the preamble data.
Now, in the normal portable telephone system of the frequency division multiple access (FDMA) method or the time division multiple access (TDMA) method, to detect the preamble data corresponding to the high priority data, the preamble data is actually decoded not by decoding but by measuring electric power of the random access channel. On the contrary, in the portable telephone system of the code division multiple access (CDMA) method which is not based on the physical division by such as frequency or time but based on the division by the difference of spread codes, other signals are intermingled on the band. Thereby, simple measurement of electric power can not detect the preamble data, and the preamble data is resultantly decoded so as to detect the preamble data.
Here, this point will be described below with a concrete example. Note that control data is also generated in one frame in this case, and the control data is transmitted by one transmission slot.
As shown in FIG. 4, as for the data structure for one frame, the preamble field is generated in the first half of the frame and the information field is in the latter half. In this case, a preamble data is stored in the preamble field, and information bits forming a message data-and error detection and correction bits of the information bits are stored in the information field.
Here, FIG. 5 shows a transmitter for actually transmitting control data of such a data structure. As shown in FIG. 5, in the transmitter 20, a bit stream DP1 of preamble data is firstly input to a frame generation circuit 21, and a bit stream DM1 of message data is input to an error correction bit adding circuit 22.
The error correction bit adding circuit 22 calculates error detection and correction bits based on the input bit stream DM1, and adds them to the bit stream DM1 so as to generate a bit stream D11 to be stored in the aforesaid information field. The bit stream D11 is output to the frame generation circuit 21. The frame generation circuit 21 adds the bit stream D11 to the end of the bit stream DP1 of the preamble data as shown in FIG. 4 so as to generate a transmission data stream D12 for one frame, which is output to a modulation circuit 23.
The modulation circuit 23 performs a predetermined modulation processing on the transmission data stream D12 to generate a transmission symbol stream D13, which is output to a transmission circuit 24. After multiplying the transmission symbol stream D13 by a desired spread code and performing the filtering processing, the transmission circuit 24 performs the digital-to-analog conversion processing to generate a transmission signal. The transmission circuit 24 then performs the frequency conversion processing on the transmission signal to generate transmission signal S10 of a predetermined band. The transmission signal S10 obtained in this way is transmitted via an antenna 25, so that the transmitter 20 transmits the control data composed of preamble data and message data.
On the other hand, in a receiver 30, the transmission signal S10 transmitted from the transmitter 20 is received at an antenna 31, and is input to a reception circuit 32 as a reception signal S11, as shown in FIG. 6. After performing the filtering processing on the reception signal S11, the reception circuit 32 performs the frequency conversion processing on the reception signal S11 to take out a baseband signal, and performs the analog-to-digital conversion processing on the baseband signal to take out a reception symbol stream D14.
The demodulation circuit 33 performs a predetermined demodulation processing on the reception symbol stream D14 taken out by the reception circuit 32 to decode a reception data stream D15 (the reception data stream D15 is not completely equal to the transmission data stream D12, and includes data errors received through transmission), which is output to a field division circuit 34.
The field division circuit 34 divides the decoded reception data stream D15 into a bit stream D16 of preamble field and a bit stream D17 of information field, which are output to a preamble detection circuit 35 and an error detection and correction circuit 36 respectively. In connection, the field division circuit 34 divides the field by the time division method. More specifically, as shown in FIG. 4, the preamble field is temporally earlier than the information field, so that the field is divided by using this timing.
The preamble detection circuit 35 judges whether the bit stream D16 is a preamble data or not. If it is preamble data, the preamble detection circuit 35 outputs a control signal S12 to the error detection and correction circuit 36. The error detection and correction circuit 36 starts the error detection and correction processing to detect data errors included in the information bits of the received message data based on the error detection and correction bits included in the input bit stream D17, and corrects the data errors. As a result, if the information bits of the message data are correctly decoded, the error detection and correction circuit 36 outputs a bit stream DM2 of message data to a control circuit (not shown) for controlling communication sequence, etc. This allows the control circuit to recognize the reception of message data and control the communication sequence in accordance with the message data.
In connection, it has been described that the field is divided by the field division circuit 34. However, there is also another case where the preamble data at the head of the reception data D15 are simply detected without the field division for separating data, and if the preamble data is detected as a result, the error correction processing of next message data is performed.
In the conventional communication method, when transmitting a combination of a high priority data and a low priority data, although each data is processed individually, a processing of the same level is performed just separately without reflecting the priority.
In addition, in the conventional communication method, when transmitting control data using the random access channel for example, a preamble data is detected to confirm the existence of message data. This processing is accomplished easily since the preamble data and the message data are divided in the time direction. However, in communication of the multi-carrier method in which data to be transmitted are transmitted at the same time using a plurality of carriers, there is a problem that such processing in time direction can not be performed since the preamble data and the message data are not divided in time direction.
Accordingly, in the case where a control data composed of a preamble data and a message data are transmitted by the multi-carrier method communication, it can be generally considered that the signal components of the preamble data and the signal components of the message data are extracted by dividing them in frequency direction at the receiving side, so that the respective data are decoded. However, this method needs a high-precision filter for dividing the signal components of the preamble data and the signal components of the message data, so that the configuration of the device at the receiving side tends to be complicated.